NEW YORK - A tropical wave in the central Atlantic Ocean was a little better-defined early Thursday and could become the sixth tropical depression of the season in the next day or two, the U.S. National Hurricane Center said in its morning advisory.

A tropical depression is a tropical cyclone in which the maximum sustained wind speed is 38 mph or less.

The NHC said upper-level winds were expected to be favorable for development of the system, which was located about 600 miles

east of the Windward Islands and generally tracking westward.

Computer models still show the system eventually moving into the Caribbean.

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Energy traders were keeping a close eye on the tropical wave, which could force short sellers to cover ahead of the long U.S. Labor Day holiday weekend amid fears a stronger storm could develop in warmer Caribbean waters and possibly threaten U.S. Gulf of Mexico oil and natural-gas production.